Sometimes they're just goofing off. They'll also shoot about 4 feet into the air, take off flying across the yard, cackling as they do so. Younger birds do it more frequently, but some of my older hens play too. Or they're chasing bugs. If it seems aimless, more likely it's play.
Dave it's chickens being chickens sometimes they feel the need to go over there fast and sometimes they don't. I have a mixed flock and it seems like the bigger heavier one do it more often than the smaller faster ones, it seems like their trying to keep up or beat the more active ones to that bug .